﻿using System;
using System.Diagnostics;
using System.Runtime.CompilerServices;

namespace JpegLibrary
{
    internal static class JpegZigZag
    {
        private static ReadOnlySpan<byte> s_blockToBuffer => new byte[]
        {
            0, 1, 5, 6, 14, 15, 27, 28,
            2, 4, 7, 13, 16, 26, 29, 42,
            3, 8, 12, 17, 25, 30, 41, 43,
            9, 11, 18, 24, 31, 40, 44, 53,
            10, 19, 23, 32, 39, 45, 52, 54,
            20, 22, 33, 38, 46, 51, 55, 60,
            21, 34, 37, 47, 50, 56, 59, 61,
            35, 36, 48, 49, 57, 58, 62, 63
        };

        private static ReadOnlySpan<byte> s_bufferToBlock => new byte[]
        {
            0, 1, 8, 16, 9, 2, 3, 10,
            17, 24, 32, 25, 18, 11, 4, 5,
            12, 19, 26, 33, 40, 48, 41, 34,
            27, 20, 13, 6, 7, 14, 21, 28,
            35, 42, 49, 56, 57, 50, 43, 36,
            29, 22, 15, 23, 30, 37, 44, 51,
            58, 59, 52, 45, 38, 31, 39, 46,
            53, 60, 61, 54, 47, 55, 62, 63
        };

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int BlockIndexToBuffer(int index)
        {
            Debug.Assert((uint)index < 64);
            return s_blockToBuffer[index];
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int BufferIndexToBlock(int index)
        {
            Debug.Assert((uint)index < 64);
            return s_bufferToBlock[index];
        }
    }
}
